Recently, electronic products have been improved to be lighter in weight, smaller in size, and higher in density, thereby resulting in an increase in the demand for various types of substrates for printed wringing boards, especially, the demand for flexible laminate (which may be referred to as flexible printed wiring boards (hereinafter FPCs) or the like later). The flexible laminate has such a structure that a circuit is formed with a metal foil on an insulating film.
A flexible laminate is generally produced by adhering a metal foil on a substrate by heat and pressure applications with an adhesive material of various kinds therebetween. The substrate is an insulating film being flexible and made of an insulating material of various kinds. The insulating film is preferably a polyimide film or the like. The adhesive material is generally a heat-curable adhesive agent, which is epoxy-based, acryl-based, or the like. (The FPC in which such a heat-curable adhesive agent is used may be refereed to as a three-layered FPC).
The heat-curable adhesive agent is advantageous in that it allows adhesion at a relatively low temperature. However, it is expected that the three-layered FPCs cannot sufficiently satisfy future demands for better heat resistance, flexibility, and electric reliability. In view of this, FPCs in which a metal layer is formed directly on the insulating film, or in which the adhesive layer is made of a thermoplastic polyimide have been proposed (hereinafter, these FPCs may be referred to as two-layered FPCs). The two-layered FPCs show better characteristics than the three-layered, and higher demands for the two-layered FPCs are expected.
Polyimide films are generally poor in adherability with polyimide-based adhesives made of thermoplastic polyimide, and requires surface-roughing treatment (such as plasma treatment, corona treatment, or the like), treatment such as adding a coupling agent, a particular metal composition, or the like in order to attain high adherability. This leads to high cost, or deterioration in film property (see Patent Citations 1 to 3).    (Patent Citation 1) Japanese Patent Application Publication, Tokukaihei, No. 5-222219.    (Patent Citation 2) Japanese Patent Application Publication, Tokukaihei, No. 6-32926.    (Patent Citation 3) Japanese Patent Application Publication, Tokukaihei, No. 11-158276.